Fabricating a high density EPROM cell by removing a portion of the field insulator regions

ABSTRACT

An EPROM disclosed in this specification includes a unique floating gate memory cell which may be charged using a reduced voltage level. The memory cells are fabricated using a mask to define the buried source, drain, and field oxide regions of the memory cell. After removal of the mask, field oxide regions are formed and a floating gate is fabricated which extends beyond the boundaries of the channel region for the floating gate field effect transistor memory cell. This extended floating gate provides additional capacitive coupling between the gate/word line and the floating gate while maintaining the same capacitive coupling between the floating gate and the channel of the floating gate field effect transistor memory cell. One embodiment discloses a silicide which is applied to the buried source and drain regions. The silicide is fabricated by forming a slot through the field oxide, forming a silicide on the diffused regions, refilling the slot with an oxide, and planarizing the resulting structure.

This is a divisional of application Ser. No. 07/966,615, filed Oct. 26,1992; which is a continuation of application Ser. No. 07/707,242 filedMay 22, 1991 now abandoned; which is a continuation of application Ser.No. 07/526,961 filed May 22, 1990 now abandoned; which is a continuationof application Ser. No. 07/128,549 filed Dec. 3, 1987, now abandoned.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuit fabrication.More specifically, this invention relates to the field of electricallyprogrammable read only memories and their fabrication.

BACKGROUND OF THE INVENTION

One goal in the fabrication of integrated circuitry is to produce acircuit having maximum circuit density. More succinctly, the goal is toprovide more circuit capability in a smaller circuit surface area. Thisgoal extends to the fabrication of EPROMs. An EPROM is a read onlymemory device in which stored data may be erased and new data written inits stead. A widely used type of EPROM is the floating gate field effecttransistor type.

A partial schematic diagram of an EPROM using floating gate field effecttransistors is shown in FIG. 1. Memory cells 26-1-1 through 26-2-4 arefloating gate field effect transistors. Row decoder 28 provides outputsignals on row lines 24-1 and 24-2 in response to signals provided onrow address input leads 21 and from read/write indicator 23. Columndecoder 29 provides and receives signals on column lines 25-1 through25-5 in response to signals provided on column address input leads 22and from read/write indicator 23. A memory output signal is provided onoutput lead 27. A data bit stored in, for example, memory cell 26-1-1 isread by providing a high voltage output signal on row line 24-1 andproviding a low voltage output signal on all other row lines. Columndecoder 29 then senses, via column lines 25-1 and 25-2, the impedance ofmemory cell 26-1-1. If the floating gate of memory cell 26-1-1 containsexcess electrons, the negative charge of these excess electrons raisesthe threshold voltage of memory cell 26-1-1 so that the voltage providedon row line 24-1 is insufficient to cause the channel of memory cell26-1-1 to conduct. Therefore, column decoder 29 detects a high impedanceand provides an appropriate signal on output lead 27. If there are noexcess electrons stored on the floating gate of memory cell 26-1-1, thenthe voltage supplied on row line 24-1 is sufficient to cause memory cell26-1-1 to conduct. Therefore, column decoder 29 detects a low impedanceand provides the appropriate signal on output lead 27.

EPROM 20 is thus programmed by negatively charging the floating gate ofselected memory cells. This is accomplished by injecting electronsthrough the insulating layer between the floating gate and the substrateof the memory cell. One fact of particular importance in understandingthe present invention is the relationship between this injection and theelectric field from the floating gate to the channel of a floating gatefield effect transistor. The greater the field between the floating gateand the channel of the floating gate field effect transistor, thegreater the injection or discharge current, depending upon the polarityof the electric field.

One prior art method for fabricating an EPROM that includes floatinggate field effect transistor memory cells is described in McElroy, U.S.Pat. No. 4,373,248, entitled "Method of Making High DensitySemiconductor or the Like", issued Feb. 15, 1983, and assigned to theassignee of the present invention. As shown in FIGS. 8A-8F thereof, thefloating gate patterns and defines the channel area of the floating gatefield effect transistor memory cells. Experimental evidence has shownthat the EPROM cell of McElroy requires a voltage level of approximately18 volts (for a floating gate to substrate insulator of silicon dioxidehaving a thickness of 350 angstroms and an interpoly insulator composedof 250 angstroms of silicon nitride and 250 angstroms of silicondioxide) on the control gate (row line) to efficiently transfer chargethrough the insulator from the channel to the floating gate. Thisvoltage limits the extent to which the EPROM cell of McElroy can bereduced in size because that voltage level requires certain spacingbetween active elements in order to avoid breakdown currents andunwanted field effects in the EPROM. Therefore, it is desirable toprovide an EPROM cell which may be programmed using a minimum voltagelevel.

Moreover, a conventional buried diffusion which forms at least a portionof a column line 25 (see FIG. 1) may exhibit a resistance of around 30ohms per square. This amount of resistance is typically too high for theburied diffusion alone to operate as a column line. Thus, memory arraystypically include a metallic strapping line which parallels and overliesa buried diffusion line to lower this resistance. A contact between theburied diffusion and metallic strapping line is provided for each of apredetermined number of memory cells. As a result, a column lineincludes a metallic strapping line electrically in parallel with aburied diffusion line, and the overall impedance of a column linegreatly decreases compared to the impedance achievable with the use ofonly a buried diffusion line.

However, semiconductor substrate area must be dedicated to providing thecontacts between the metallic strapping lines and the buried diffusionlines. This contact area lowers the memory array cell density over thedensity which could be achieved if some of the contact areas couldinstead be used for memory cells. Therefore, it is desirable to providean EPROM cell which requires fewer contacts between buried diffusionlines and metallic strapping lines.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that a highcapacitance coupling is provided between floating gates and controlgates of memory cell transistors. A high capacitance coupling permitsthe use of lower voltages during programming, and spacing between activeelements decreases as a result.

Another advantage of the present invention is that a silicide layer isformed on buried column lines. Consequently, the resistance of a buriedcolumn line decreases, and fewer contacts between buried column linesand metallic strapping lines are required. In addition, the decreasedresistance lowers an overall RC time constant associated with readoperations of the EPROM. Faster read operations result.

The above and other advantages of the present invention are carried outin one form by an EPROM having a unique floating gate memory cell whichmay be charged using a reduced voltage level. The memory cell isfabricated using a mask to define buried source/drain and field oxideregions of the memory cell. A floating gate is fabricated which extendsbeyond the boundaries of the channel region for the floating gate fieldeffect transistor memory cell. This extended floating gate providesadditional capacitive coupling between the gate/word line and thefloating gate while maintaining substantially the same capacitivecoupling between the floating gate and the channel of the floating gatefield effect transistor memory cell. This arrangement provides a greaterelectric field between the floating gate and the channel of the floatinggate field effect memory transistor cell for given voltage levels on thegate, source, and drain of the floating gate field effect transistormemory cell. Therefore, a lower voltage level may be used to write datainto the EPROM cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an EPROM using floating gate fieldeffect transistors;

FIGS. 2A through 2G are schematic side-view diagrams depicting theprocessing steps necessary to fabricate a floating gate field effecttransistor memory cell according to the teachings of this invention;

FIG. 3 is a plan view of an intermediate step in the process illustratedby FIGS. 2A through 2G;

FIG. 4 is a plan view of a portion of an EPROM fabricated according tothe teachings of this invention; and

FIGS. 5A through 5F show side-view diagrams that illustrate theprocessing steps performed to fabricate a silicided, buried column line.

DETAILED DESCRIPTION

FIGS. 2A through 2G are side-view schematic diagrams depicting theprocessing steps for fabricating an EPROM according to one embodiment ofthe present invention. The process of this embodiment begins with aP-epitaxial layer 1b formed on a P+ substrate la as shown in FIG. 2A. Aninitial oxide layer 2 is then formed by thermal oxidation in a steamambient at 900° C. for approximately 10 minutes to a thickness ofapproximately 350 angstroms. A silicon nitride layer 5 is then formed onthe surface of oxide layer 2 by low pressure chemical vapor deposition(LPCVD) to a thickness of approximately 1000 angstroms. Silicon dioxidelayer 2 and silicon nitride layer 5 are then patterned and etched usinga technique well known in the art. The resulting structure, shown inFIG. 2A, includes etched portions which generally correspond to columnlines 25 (see FIG. 1).

FIG. 2A shows the initial step for fabricating two adjacent floatinggate field effect transistor memory cells. A complete EPROM will containany number of memory cells limited only by fabrication techniques andthe surface area of substrate 1a. Thus, although FIGS. 2A through 2Gdescribe the fabrication of two memory cells, it is understood that manymore memory cells are fabricated in conjunction with the two cells shownin FIGS. 2A through 2G and that the cells extend to both the left andright-hand side of the page and in the plane perpendicular to the pageboth into the page and out of the page.

The structure of FIG. 2A is next subjected to an implantation of arsenicions having an energy of approximately 50 kiloelectron volts and adensity of approximately 1E16 ions/cm², and/or phosphorus ions having anenergy of around 100 kiloelectron volts and a density of about 1E15ions/cm². This ion implantation is then annealed in a nitrogenenvironment at approximately 900° C. for approximately 100 minutes.Field insulation regions, such as field oxide regions 6, may then begrown by thermal oxidation in a steam environment at approximately 850°C. for approximately 20 minutes to a thickness of approximately 4,000angstroms as shown in FIG. 2B. N++ regions 7, 8, and 9 represent regionswhere ion implantation occurred. Thus, regions 7, 8, and 9 function asthe source/drain regions of the floating gate field effect transistormemory cells. Silicon nitride layer 5 may next be deglazed and removedusing techniques well known in the art.

In a second embodiment, a stack has a silicon dioxide layer 4 and apolycrystalline silicon layer 3 formed between silicon dioxide layer 2and silicon nitride layer 5, as shown in FIG. 2C. This stack is used topattern field oxide regions 6 and N++ source/drain regions 7, 8, and 9(see FIG. 2B). Polycrystalline silicon layer 3 is included to provide afield plate periphery isolation region (not shown) for the EPROM. Ifmore conventional isolation methods, such as trench or field oxideisolation, are used, polycrystalline silicon layer 3 and silicon dioxidelayer 4 may be omitted.

In a third embodiment, the oxidation mask may be omitted entirely and asimple photoresist mask (not shown) may be used to pattern the ionimplantation for N++ source/drain regions 7, 8, and 9. After removingthe photoresist mask, the field oxide regions are thermally grownwithout an oxidation mask. The increased oxidation rate of doped siliconat a temperature of approximately 850° C. results in an oxide growthrate of approximately 8-10 times the growth rate for undoped silicon.Thus, after this oxidation step, thick field oxide regions 6 reside overN++ source/drain regions 7, 8, and 9, while a much thinner oxide isformed over the undoped regions. This thin oxide layer may be used for agate oxide such as layer 30 shown in FIG. 2D and discussed below.

It will be understood by those skilled in the art that field insulatorregions, such as field oxide regions 6, represent substantiallydifferent structures than gate oxides. Field insulator regions typicallyseek to capacitively isolate regions on opposing sides thereof.Consequently, oxides used over P+ substrates for field insulator regionsare typically greater than 800 angstroms thick in currently manufacturedsemiconductor devices. On the other hand, gate oxides must permitcoupling through a gate oxide in order for a transistor to properlyfunction. Thus, gate oxides are typically less than 800 angstroms thickin currently manufactured semiconductor devices.

Returning to the initial embodiment, after removing silicon nitridelayer 5, the structure shown in FIG. 2D remains. The structure of FIG.2D may be subjected to a silicon dioxide etch using techniques wellknown in the art to strip silicon dioxide between field oxide regions 6.Thermal oxidation may then be performed in an oxygen ambient including 5percent hydrochloric acid at a temperature of approximately 900° C. forapproximately 45 minutes to provide gate oxide layers 30 between fieldoxide regions 6. Gate oxide layers 30 are approximately 300 angstromsthick.

A polycrystalline silicon (poly) layer 10 is deposited using lowpressure chemical vapor depositing to a thickness of approximately 2,000angstroms, as shown in FIG. 2E. Poly layer 10 is doped by diffusingPOC13 for approximately 10 minutes at a temperature of approximately950° C. Poly layer 10, is then patterned and etched to provide thestructure shown in FIG. 2E. At this point poly layer 10 is partiallypatterned to provide strips 10A, as shown in FIG. 3. Strips 10A arepatterned into individual floating gates when the control gates/wordlines are patterned in steps described below and as shown in the planview of FIG. 4.

In the structure of FIG. 2E poly layers 10 extend well beyond the edgesof field oxide regions 6. This structure differs from the prior art asshown in McElroy which provides floating gates that extend only to theedge of field oxide regions. The maximum extent of the overlap of fieldoxide regions 6 by poly regions 10 is limited only by fabricationtolerances required between adjacent poly regions 10.

As a minimum, such overlaps may be characterized as extendingsubstantially onto field oxide regions 6. Thus, the minimum extension offloating gates over the field insulator regions is greater than theminor amount of overlap that might possibly result from the diffusion ofa floating gate self-aligned to a field oxide region, as taught inMcElroy. This minimum extension may, for example, represent an overlapof one-tenth or more of the field insulator region. In the preferredembodiment field oxide regions 6 are approximately 3 microns wide, andthe floating gates overlap each field insulator region for a distance ofapproximately 1 micron.

Sidewall oxide filaments 42 may be formed on the edges of poly layer 10which were exposed in the previous etching step. Filaments 42 seal suchedges against electrical leakage and provide smoothing that preventsformation of harmful filaments in a later-occurring poly depositionstep. Next, poly layer 10 is deglazed and silicon dioxide layer 11 isdeposited overlying poly layer 10 using low pressure chemical vapordeposition at a temperature of approximately 800° C. to a thickness ofapproximately 250 angstroms, as illustrated by FIG. 2F. Silicon nitridelayer 12 is then formed on layer 11 by low pressure chemical vapordeposition at a temperature of approximately 800° C. to a thickness ofapproximately 250 angstroms.

The combination of silicon dioxide layer 11 and silicon nitride layer 12is used as a floating gate to active gate insulator in order to increasethe dielectric constant over, for example, an insulator of silicondioxide only. The increased dielectric constant provides increasedcapacitance between the floating gate and the active gate when comparedto capacitance achieved by the use of a silicon dioxide dielectricalone.

The structure is then subjected to thermal oxidation in a steam ambientat a temperature of 1000° C. for 20 minutes to seal the surface ofsilicon nitride film 12. Polycrystalline silicon (poly) layer 13 is nextdeposited by low pressure chemical vapor deposition to a thickness ofapproximately 4,000 angstroms. Poly layer 13, silicon nitride layer 12,silicon dioxide layer 11 and poly layer 10 are then patterned usingtechniques well known in the art. The remaining portions of poly layer13 represent conductors running parallel to the plan of the drawing.These parallel conductors provide the row lines 24-1, 24-2, etc. (SeeFIG. 1) of the EPROM. Buried N++ source/drain regions 7, 8 and 9 areparallel buried conductors which run perpendicular to the page. Theseprovide column lines 25-1, 25-2, etc. (FIG. 1). The remaining portionsof poly layer 10 represent individual floating gates for each EPROMcell.

A layer of silicon dioxide (not shown) is grown on the surface of polylayer 13 using an oxygen ambient at a temperature of approximately1,000° C. for approximately 30 minutes to a thickness of approximately500 angstroms. This silicon dioxide layer encapsulates poly layers 10and 13 in a high quality thermal oxide. Another layer of silicondioxide, which is undoped, may be formed on this silicon dioxide layeror formed separately by tetra ethyl ortho silicate (TEOS) low pressurechemical vapor deposition to form refill silicon dioxide layer 14, asshown in FIG. 2G. A layer of phosphorus-doped, or boron andphosphorus-doped, silicon dioxide 15 is then deposited by atmosphericpressure chemical vapor deposition to a thickness of approximately10,000 angstroms. Refill oxide layer 14 provides a layer of undopedsilicon dioxide which keeps the phosphorus in silicon dioxide layer 15from doping active regions of the EPROM. A layer of aluminum is thensputtered on the surface of silicon dioxide layer 15 to a thickness ofapproximately 10,000 angstroms. This aluminum layer is then patternedand etched using techniques well known in the art to provide aluminumcolumn leads 16, 17 and 18. Column leads 16, 17, and 18 run parallel toeach buried N++ diffusion, such as buried N++ source/drain regions 7, 8and 9, as shown in FIG. 2G. The entire integrated circuit is then sealedwith a protective overcoat of phosphorus doped silicon dioxide (notshown) deposited using atmospheric pressure chemical vapor deposition toa thickness of approximately 10,000 angstroms. This protective oxidelayer is then etched to provide contact points for the integratedcircuit.

FIG. 4 is a plan view of a portion of an EPROM constructed in accordancewith the teachings of this invention. Contact points 19, 20 and 21 areopenings which permit contact between metal regions 16, 17, and 18 toN++ source/drain regions 7, 8, and 9, respectively. Buried source/drainregions 7, 8, and 9 cannot be used as a sole conductor for the columnleads of an EPROM because of their high resistance which is typicallyabout 30 ohms/square, but may be lower if a particular conductive regionis incorporated as discussed below. Regardless of whether a conductiveregion is used, metal regions 16, 17, and 18 periodically contact N++source/drain regions 7, 8, and 9, respectively, in order to lower theoverall resistance of the column lead formed by, for example, metalregion 17 in conjunction with N++ source/drain region 8. Cut-away pointAA of FIG. 4 shows the region from which the side views shown in FIGS.2A through 2G are taken. It will be understood that fewer of suchcontacts are required when the conductive region discussed below isincorporated.

A floating gate field effect transistor memory cell constructedaccording to the teachings of this invention provides much greatercapacitive coupling between polysilicon region 13 and floating gates 10than between floating gates 10 and epitaxial layer 1b. This structurecan be modeled electrically by a series of two capacitors. The voltageacross a capacitor is given by the equation,

    V=Q/C, where                                               (1)

V is equal to the voltage across the capacitor,

Q is equal to the charge on the capacitor, and

C is equal to the capacitance of the capacitor.

With capacitors in series, Q is equal for all capacitors. The proportionof the total voltage across both capacitors in series is solved from theequation,

    V1/Vt=(Q/C1)/(Q/C1+Q/C2), where                            (2)

V1 is the voltage across capacitor 1,

Vt is equal to the total voltage across both capacitors,

C1 is the capacitance of the first capacitor, and

C2 is the capacitance of the second capacitor.

Solving this equation we find that,

    V1/Vt=C2/(C1+C2).

Therefore, the voltage drop for a given total voltage across onecapacitor can be raised by increasing the capacitance of the othercapacitor. By raising the capacitance between polysilicon gate 13 andfloating gates 10, the voltage between polysilicon regions/floatinggates 10 and epitaxial layer 1b is increased for a given voltage betweenepitaxial layer 1b and poly silicon layer 13. Empirical evidence hasshown that a floating gate field effect transistor memory cellconstructed according to the teachings of this invention requires awriting voltage level approximately 30 percent smaller than thatrequired in the cell described in McElroy. This allows the EPROM cellsand their drive circuitry to be reduced in size accordingly.

As discussed above, the present invention additionally contemplates anembodiment which uses conductive regions immediately overlying N++regions 7, 8, and 9. These conductive regions lower the sheet resistanceof N++ regions 7, 8, and 9. FIGS. 5A through 5F illustrate a procedurewhich a preferred embodiment utilizes to form these conductive regions,which consist of buried silicide layers. The process depicted in FIGS.5A through 5F may be inserted into the above described process startingwith the structure shown in FIG. 2D. Then, the process illustrated inFIGS. 5A through 5F may be utilized in lieu of the process discussedabove in connection with FIG. 2E to form the buried silicide layers.

Accordingly, FIG. 5A shows the structure of FIG. 2D with the addition ofa photoresist mask thereon. Specifically, the photoresist mask containssolid portions 32a, and gaps 32b. Gaps 32b overlie central portions offield oxide regions 6, and solid portions 32a overlie thin oxide regions30 and the portions of field oxide 6 which reside near the boundarybetween field oxide 6 and thin oxide 30. In the preferred embodiment,field oxide 6 is approximately 3 microns wide. Furthermore, gaps 32b areapproximately 2 microns wide and centered overlying thick oxide 6 sothat centers of gaps 32b substantially overlie centers of field oxideregions 6.

Next, field oxide regions 6 are anisotropically etched in the areasdefined by openings 32b. The etching removes portions of field oxideregions 6 until the etching causes diffused regions 7, 8, and 9 tobecome exposed. FIG. 5B shows slots 33 which are formed as a result ofthis etching. In the preferred embodiment, field oxide regions 6 exhibita thickness of approximately 4,000 angstroms. Thus, this etching stepremoves approximately 4,000 angstroms of thermally grown oxide.

Next, photoresist mask 32a-32b is removed and a layer of a refractorymetal 34, such as titanium, tungsten, molybdenum, tantalum, hafnium,vanadium, and the like, is sputtered overlying the surface of thestructure. The preferred embodiment applies titanium at 150° C. until athickness of approximately 1,000 angstroms is achieved. The resultingstructure is shown in FIG. 5B.

Referring to FIG. 5C, an approximately 675° C. nitrogen environment for30 minutes is used to form a metallic silicide 36. Silicide 36 formswherever refractory metal 34 (see FIG. 5B) contacts silicon. Thus,silicide 36 forms within slots 33 overlying N++ regions 7, 8 and 9. Inthe preferred embodiment, the 1,000 angstrom thickness of refractorymetal 34 combines with approximately 1,000 angstroms of silicon from theN++ diffusion regions 7, 8 and 9. Consequently, the resulting silicide36 is approximately 2,000 angstroms thick. In the preferred embodiment,buried diffusion regions 7, 8 and 9 are each approximately 5,000angstroms thick prior to the formation of silicide 36. Thus, N++ regions7, 8 and 9 now exhibit a thickness of approximately 4,000 angstromsunderlying silicide 36.

After the formation of silicide 36, an annealing step is performed atapproximately 800° C. for approximately 30 minutes. Then, unreactedportions of refractory metal 34 (See FIG. 5B) are removed from oxideareas 6 and 30. The structure shown in FIG. 5C results.

Referring to FIG. 5D, the process next deposits a conformal oxide coding38, such as the TEOS oxide discussed above, to a depth of approximately5,000 angstroms overlying the entire surface of the structure. The 5,000angstrom thickness of conformal oxide 38 fills slots 33. After theapplication of conformal oxide layer 38, a glass layer 40 is spun-on ina manner well-known to those skilled in the art to provide a planarsurface overlying the structure. FIG. 5D illustrates the resultingstructure after the application of glass layer 40.

Referring to FIG. 5E, an etch-back step is next performed to removeunneeded oxides from the surface of the structure and to exposeP-silicon layer 1b. An etch chemistry used in this etching step etchesglass layer 40, deposited oxide layer 38, and field oxide 6 atapproximately equal rates. However, conventional etch chemistries knownto those skilled in the art may etch thermally grown oxide layer 6 at aslightly slower rate than glass layer 40 and deposited oxide layer 38without generating undesirable consequences. Nevertheless, the resultingethed surfaces of oxide layer 6 and oxide layer 38 are approximatelycoplanar with the surface of substrate layer 1b.

The resulting structure from this etch-back step is illustrated in FIG.5E. The fabrication of the field insulator regions is now complete. Eachof these field insulator regions includes both a thermally grown fieldoxide 6 and deposited oxide 38. The field insulator regions exhibit athickness of approximately 2,000 angstroms.

It will be understood by those skilled in the art that while thethickness of the field insulator region is reduced in the embodimentshown in FIG. 5E from that depicted in FIG. 2E, the overall isolationfunction of this field insulator region may in fact be improved. Theresistance portion of the RC time constant characterizing the couplingbetween diffusions 7, 8 and 9 and overlying poly layers may be reducedby, for example, a factor of 10 or more due to the inclusion of silicide36. Thus, while the narrowing of the field insulator region may tend toincrease capacitance by a factor of around two to four, a reduction inresistance by a factor of 10 achieves a significantly reduced overall RCtime constant.

Referring now to FIG. 5F, a process step is shown which is similar tothe process step discussed above in connection with FIG. 2E.Specifically, thin oxide region 30 is regrown to form a gate oxide, andpoly layer 10 is deposited overlying thin oxide 30 and the fieldinsulator regions, as discussed above. Poly layer 10 is applied to athickness of approximately 3,000 angstroms. Next, poly layer 10 is dopedwith POC13 at approximately 950° C. so that a sheet resistance ofapproximately 30 ohms/square is achieved. Next, poly layer 10 ispatterned and etched as discussed above and shown in FIG. 3. Sidewalloxide filaments 42 may then be grown on exposed sidewalls of poly layer10.

In the preferred embodiment, the field insulator region, which consistsof thick oxide 6 and deposited oxide 38, is approximately 3 micronswide. Poly layer 10 overlaps each of the field insulation regions byapproximately 1 micron. Thus, a one micron gap between strips of polylayer 10 exists centrally located on each of the field insulatorregions.

The process of the present invention then continues substantially asdiscussed above in connection with FIGS. 2F through 2G. However, thoseskilled in the art will recognize that the field insulator regions andunderlying structures beneath poly layer 10 will resemble the structureshown in FIG. 5F rather than the specific structure shown in FIGS. 2Fthrough 2G.

Accordingly, a transistor memory cell array may be constructed toprovide a decreased sheet resistance for buried diffusions. A typicalresistance of buried diffusions 7, 8 and 9 alone may be approximately 30ohms/square. A typical sheet resistance of silicide 36 constructedaccording to the process discussed above would be approximately 3ohms/square. A combination of silicide 36 with diffusions 7, 8 and 9forms a structure which is electrically equivalent to two conductors inparallel with each other. Consequently, the combined resistance ofsilicide 36 and buried diffusions 7, 8, and 9 is slightly less than 3ohms/square. An approximately 10-fold decrease in resistance has beenachieved by the addition of silicide 36.

In summary, the present invention provides a method whereby EPROM memorycells may be programmed using a reduced voltage. The use of a reducedvoltage permits the use of smaller memory cells in the construction ofan EPROM. In addition, one embodiment of the present invention providesa method for reducing the sheet resistance of buried diffusions, such asthe buried diffusions which form column lines of an EPROM memory array.The reduced resistance permits the use of fewer contacts between theburied diffusions and overlying metallic layers. Consequently,semiconductor substrate area may be used for memory cells rather thansuch contacts.

The foregoing description uses various embodiments to illustrate thepresent invention. However, those skilled in the art will recognize thatchanges and modifications may be made in these embodiments withoutdeparting from the scope of the present invention. For example, theprecise dimensions disclosed herein may vary widely. In addition,substantially equivalent structures to those described herein may beobtained using processes which differ slightly from those describedabove. These and other modifications obvious to those skilled in the artare intended to be included within the scope of this invention.

What is claimed is:
 1. A method for forming an electrically programmablememory cell on a substrate having a semiconductor surface area of afirst conductivity type, said method comprising the steps of:applying amask on the surface of said substrate; applying dopant ions of a secondconductivity type to said substrate in areas of said substrate notcovered by said mask; removing said mask; forming field insulatorregions above said ions; forming a floating gate overlying saidsubstrate surface partially over said field insulator regions; formingan interlevel insulator on a surface of said floating gate; forming anactive gate on a surface of said interlevel insulator so that acapacitance exists between said active gate and said floating gate;removing a portion of each of said field insulator regions to form fieldinsulator region slots; and forming a conductive region on saidsubstrate surface within said slots.
 2. A method as claimed in claim 1wherein said forming a conductive region step comprises the step ofapplying a refractory metal to said substrate surface within said slots.3. A method as claimed in claim 1 additionally comprising the step offorming an insulating region overlying said conductive region in saidslots.
 4. A method as claimed in claim 1 wherein said forming a floatinggate step comprises the step of extending said floating gate to overlieportions of said slots.